The port is located just north of downtown Miami, near the Bayside Marketplace & American Airlines Arena. There is plenty of parking available adjacent to the terminal, and easily accessible. We have stayed nearby at the Marriott Biscayne Bay, and on South Beach at the new Marriott South Beach. The Biscayne Bay is closer and less expensive. Staying close and parking your car will be a breeze. We have done it several times with no problems.
The Mariott which was mentioned above over-looks the Port of Miami w/ excellent views of incoming and outgoing ships. We're staying there next time we cruise. Also, the parking at the port is very nice as it's covered. The price is $10 per day. You'll have no trouble finding the port. The closer you get to downtown the more likely you will see the tops of some of the ships.
What ship are you going on? We just returned from the Explorer. It was wonderful.